1. Field of the Invention
The present invention relates to a digital receiver, and more particularly, to an Advanced Television Systems Committee (ATSC) digital television (DTV) receiver including a digital surface acoustic wave (SAW) filter and/or a digital gain control circuit to remove the influence of an adjacent-channel's interference signal.
2. Description of the Related Art
An ATSC receiver is a digital television (DTV) receiver that can receive signals transmitted according to the ATSC DTV standard A/53, detailed information about which can currently be obtained at http://www.atsc.org. This standard is approved by the Federal Communications Commission (FCC) for use in terrestrial broadcasting in the United States of America. Advanced Television Systems Committee (ATSC) digital television (DTV) broadcast signals have been transmitted by many television stations in the United States and Korea. However, since National Television Standards Committee (NTSC) broadcast signals are still used together with ATSC DTV broadcast signals, ATSC receivers need to have excellent performance despite an environment of interfering DTV broadcast signals.
A Digital Television Standard published by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals within the same 6-MHz-bandwidth television channels that are currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum interleaves with the spectrum of a co-channel interfering NTSC analog TV signals. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal that fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 Hz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel. The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238*106 symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
In a location where two or more digital TV stations influence (interfere with) each other, the magnitude of an adjacent signal of an unwanted channel may be greater than that of a signal of a wanted channel. Wireless DTV transmissions environments typically have a larger dynamic range (range of signal magnitude) than cable transmission environments. When a ratio of the magnitude of a signal of an unwanted channel to the magnitude of a signal of a wanted channel increases, it may be difficult or impossible for a user to reliably receive the signal of the wanted channel depending upon the performance of the tuner and/or receiver. Accordingly, an ATSC DTV receiver capable of removing an adjacent channel's interference signal (removing adjacent-channel interference) is desired to increase receiving performance.
In DTV receivers and similar receiver apparatus for receiving a carrier amplitude-modulated by multi-level symbol code sequences, demodulation has been done in the analog regime to reproduce the multi-level symbol code sequences at baseband, with the demodulation results subsequently being digitized. ATSC DTV receivers can be constructed in which an intermediate-frequency (IF) amplitude-modulation (AM) signal developed by down-conversion of a received radio-frequency (RF) AM signal is demodulated by synchrodyning in a mixer. It is preferable that the IF signal be supplied as a complex signal having real (Q) and imaginary (I) portions.
The amplitude and phase mismatches between the receiver I and Q signal branches may result in imperfect attenuation of the image signal band. The task of improving the image signal attenuation of the basic quadrature downconversion scheme, either using analog or digital techniques, has been addressed to some extent by conventional real image rejection filtering.
FIG. 1 is a functional block diagram of a tuner 10 having a typical dual (two) SAW (surface acoustic wave) filter structure. Referring to FIG. 1, a mixer 14 functioning as a down converter receives a radio frequency (RF) signal (received via an antenna 12) and a signal output from a local oscillator 16, and outputs an intermediate frequency (IF) signal having a center frequency of 44 MHz based on the radio frequency (RF) and the signal output from a local oscillator 16.
A first SAW filter 18 removes (filters out) an image signal and an adjacent interference signal from the IF signal output from the mixer 14. Generally, the bandwidth of the first SAW filter 18 is greater than 6 MHz. The first SAW filter 18 tends to attenuate the signal passing through it.
A first variable amplifier (AMP) 20 adjusts the gain of the signal attenuated by the first SAW filter 18 based on an automatic gain control signal AGC1 output from an automatic gain control (AGC) circuit 22. The AGC circuit 22 generates the automatic gain control signal AGC1 based on an output signal of the first AMP 20.
A second SAW filter 24 removes (filters out) an image signal and an adjacent interference signal from the output signal of the first variable amplifier 20. The bandwidth of the second SAW filter 24 is less than that of the first SAW filter 18. A second variable amplifier 26 adjusts the gain of a signal attenuated by the second SAW filter 24 based on an automatic gain control signal AGC2 output from a demodulator (not shown). An output signal of the second variable amplifier 26 is output to an analog-to-digital (ADC) (not shown). The tuner 10 may be implemented on a single chip.
FIG. 2 is a functional block diagram of a tuner 30 having a single (one) SAW (surface acoustic wave) filter structure. Referring to FIG. 2, a mixer 34 outputs an IF signal having a center frequency of 44 MHz based on an RF signal received through an antenna 32 and upon a signal output from a local oscillator 36. A SAW filter 38 removes (filters out) an image signal and an adjacent interference signal from the IF signal output from the mixer 34. The SAW filter 38 attenuates the signal it receives and filters. An variable amplifier 40 adjusts the gain of the signal attenuated by the SAW filter 38 based on an automatic gain control signal AGC2 output from a demodulator (not shown).
When necessary, the tuner 30 may further include the first variable amplifier 20 and/or the AGC circuit 22 illustrated in FIG. 1. When the single SAW filter 38 is used as illustrated in FIG. 2, the structure of the tuner 30 is simplified and development cost is reduced.